Adjustable attenuator with p-i-n diodes

ABSTRACT

An antenna and television receiver are interconnected by an adjustable attenuator in the form of a pi network having a longitudinal branch with a p-i-n diode, an antenna cross branch with high frequency short-circuiting and adjustable control means, and a receiver cross branch with high frequency shortcircuiting means and a second p-i-n diode coupled to a positive bias source whereby variable attenuation is accompanied by substantially constant input and output impedances.

United States Patent 1 Maier Nov. 5, 1974 [541 ADJUSTABLE ATTENUATORWITH P-l-N 3,624,561 11/1971 Tongue 333/81 R x DlODES 3,663,900 5/1972Peterson 333/81 R [75] Inventor. gerhard Marer, Ahornweg, m my Examinerpaul L Gensler I ermany Attorney, Agent, or FirmTheodore C. Jay, Jr.;-[73] Assignee: SABA Schwarzwalder Robert T. Orner; Thomas H. BufftonApparate-bau-Anstalt August Schiver Sohne G.m.b.H., Villingen, [57]ABSTRACT Germany An antenna and television receiver are interconnected[22] Flled: July 1973 by an adjustable attenuator in the form of arrnetwork [211 App] 2 4 having a longitudinal branch with a p-i-n diode,an antenna cross branch with high frequency shortcircuiting andadjustable control means, and a re- [52] U.S. Cl. 333/81 R, 333/81 Aceiver cross branch with high frequency Show [51] 'I 7/24, 1 1/22circuiting means and a second p-i-n diode coupled'to [58] Fleld ofSearch 333/81 R1 81 A; 323/80 a positive bias source whereby variableattenuation is accompanied by substantially constant input and out-References Cited impedances UNITED STATES PATENTS 2 Claims, llDrawingFigure 1 ADJUSTABLE ATTENUATOR WITH P-I-N DIODES BACKGROUND OF THEINVENTION The present invention relates to an adjustable attenuator withp-i-n diodes.

In order to match input signals of different strengths, for instanceantenna signals, to the input amplifier of a high frequency receiver,attenuators are connected between the antenna and the input of theamplifier. These attenuators are adjustable and their attenuation can bevaried continuously or stepwise without the input and output impedancesbeing changed; The attenuators were first of all constructed withresistors in 11' or tee networks.

The semiconductor technique created a new component, the p-i-n diode. Itconstitutes an electronically adjustable attenuator which, due to itsspecial construction, has the property of changing its HF forwardresistance as a function of the direct current flowing through it with afalling characteristic. It therefore rep resents an ideal component forthe completely electronic control of attenuation.

It is already known to arrange p-i-n diodes, similar to thecorresponding attenuators with resistors, in 71' as well as tee networks(Radio-Elekronik-Schau, Issue 9, page 497). 1r networks are particularlypreferred in order to maintain equal impedances at the input and at theoutput independently of the attenuation of the attenuator. In practice,however, it was found that the match at the antenna input changes verygreatly upon misadjustment and thus extensive reflections can occur.

It is found in an attenuator constructed with p-i-n diodes that as aresult of the p-i-n diode installed in the antenna-side cross branch ofthe 12' member, the antenna is more and more short-circuited uponadjustment of the attenuator, since the diode connected in the crossbranch is adjusted so as to be more conductive and the diode connectedin the longitudinal branch more highly ohmic. This has the result thatupon increased misadjustment of the attenuator, one operates more andmore in the antinode at the antenna jack, as a result of which ghostsappear in the television picture.

A controllable attenuator member for highfrequency signals using p-i-ndiodes is also known (German Patcnt 2,105,747). The object of this knownarrangement is to avoid nonlinearities of the p-i-n diodes operated inthis case in the region of the lower cutout frequency. The linearity ofthe p-i-n diodes for this lowfrequency range is improved in the mannerthat two p-i-n diodes connected in opposition to each other areconnected in the cross branch parallel to a load resistor. A controlcurrent is fed via a common connection to a connecting line between thetwo p-i-n diodes and removed via the outer connections facing away fromthe connecting line of the two p-i-ndiodes. With such an arrangement,however, it is not possible to obtain high values of attenuation, sincethe diodes'connected in the cross branch have their forward resistancesconnected in parallel, in which connection the attenuation can beadjusted in only one cross branch. Furthermore, such networks do notsatisfy the. requirement for matching.

OBJECTS AND SUMMARY OF THE INVENTION An object of the present inventionis to provide an p-i-n diode in a longitudinal branch interconnecting anantenna and a signal receiver, an antenna cross branch having highfrequency shortcircuiting and adjustable control means, and a receivercross branch having high frequency shortcircuiting and a second p-i-ndiode coupled to a positive bias source.

BRIEF DESCRIPTION OF THE DRAWING The sole FIGURE illustrates a preferredembodiment of an attenuation circuit for an antenna and signal receiver.

PREFERRED EMBODIMENT OF THE INVENTION,

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe accompanying drawing. 1

Referring to the drawing, a first p-i-n diode D1 is connected in thelongitudinal branch of a 11 network intermediate an antenna terminal Aand a signal receiver terminal E. An antenna cross branch includes aresistor R1 coupling the junction of the diode D1 to circuit ground byway of a high frequency short-circuiting capacitor C2. The resistor R1and capacitor C2'are coupled to the'emitter electrode of a transistor Thaving a collector coupled to a potential source and a base coupled by aresistor R2 to a variable amplifier signal source R.

A receiver cross branch includes a second p-i-n diode D2 coupled to thefirst p-i-n diode D1 and to the receiver terminal E. The second p-i-ndiode D2 is coupled to circuit ground by a high frequencyshortcircuiting capacitor C3 and to the junction of a pair of resistorsR3 and R4 connected intermediate a potential source and circuit ground.Another resistor R5 couples the junction of the p-i-n diodes D1 and D2and the receiver terminal E to the potential reference level or circuitground.

As to operation, an input signal available at the antenna input terminalA is fed via a capacitor C1 and first p-i-n diode D1 to receiverterminal E. Depending upon the control voltage applied to the base ofthe transistor T, a corresponding current flows from the potentialsource via the collector-emitter path of the transistor T, the resistorR1, the p-i-n diode D1, and the re-.

sistor R5 to circuit ground. Thus, a positive bias voltage isestablished at the cathode of the p-i-n diodes D1 and D2.

When the current flow through the resistor R5 becomes so small that itdrops below the fixed bias of the voltage divider consisting of theresistors R3 and R4, the p-i-n diode D2 will open and, with increasing.current flow, its resistance will be reduced whereupon attenuation isthus increased. Moreover, the resistor R1 in the antenna cross branch ofthe attenuator is so di mensioned that the ratio of the characteristicimpedance of the connected antenna to the value of the resistor R1 is ina ratio of approximately 1:4. Obviously, other ratios can be selected.

By the measures of the invention of connecting only one p-i-n diode D1in the longitudinal branch of one p-i-n diode D2 in the receiver-endcross branch, the attenuation in the critical frequency range at theupper end of the UHF band is only 4 db less than in the case of anattenuator constructed in the customary manner with three p-i-n diodes.This small difference is due to the fact that sucking effects can easilybe utilized at these high frequencies. In other words, the term suckingeffects" relates to the fact that the diode D2 and capacitor C3 tend toform a series resonant circuit having a very low impedance at afrequency in the UHF range. Thereupon, the junction of the diodes D1 andD2 is essentially grounded and short circuits any signal present at thejunction of the diodes D1 and D2.

Thus, the inductance of the p-i-n diode D2 as well as that of the linewith the capacitor C3 can form a wide" series resonance at any desiredfrequency portions of the UHF range. In absolute value, the attenuationat a frequency of 800 megacycles is of the order of magnitude of 30 db.

Additionally, a maximum mismatch of only 1:4 is obtained with the largerattenuation of the antenna input signal. In many tuners, this maximummismatch is by no means reached in normal operation. Thus, the indicatedmismatch is to be considered merely as a most unfavorable value when thetuner has a standing wave ratio of m l.

While there has been shown and described what is at present consideredthe preferred embodiment of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention as defined by the appendedclaims.

What is claimed is:

1. An adjustable 1r network form of attenuator comprising:

a longitudinal branch including a first p-i-n diode interconnectingantenna and signal receiver terminals;

an antenna cross branch including a resistor coupled to said antennaterminal and a transistor coupled to said resistor, to a potentialsource, and to a variable amplifier source, said antenna cross branchincluding a high frequency short circuiting capacitor coupling saidresistor and transistor to a potential reference level; and

a receiver cross branch including a second p-i-n diode coupling saidreceiver terminal to a positive potential source.

2. An adjustable 7T network form of attenuator comprising:

a longitudinal branch including a first p-i-n diode interconnectingantenna and signal receiver terminals;

an antenna cross branch including a resistor coupled to said antennaterminal and a transistor coupled to said resistor, to a potentialsource, and to a variable amplifier source; and

a receiver cross branch including a second p-i-n diode coupling saidreceiver terminal to a positive potential source, said receiver crossbranch including a resistor coupling said receiver terminal to circuitground and said positive potential source is in the form of a voltagedivider having a pair of resistors series connecting a potential sourceto a potential reference level.

1. An adjustable pi network form of attenuator comprising: alongitudinal branch including a first p-i-n diode interconnectingantenna and signal receiver terminals; an antenna cross branch includinga resistor coupled to said antenna terminal and a transistor coupled tosaid resistor, to a potential source, and to a variable amplifiersource, said antenna cross branch including a high frequency shortcircuiting capacitor coupling said resistor and transistor to apotential reference level; and a receiver cross branch including asecond p-i-n diode coupling said receiver terminal to a positivepotential source.
 2. An adjustable pi network form of attenuatorcomprising: a longitudinal branch including a first p-i-n diodeinterconnecting antenna and signal receiver terminals; an antenna crossbranch including a resistor coupled to said antenna terminal and atransistor coupled to said resistor, to a potential source, and to avariable amplifier source; and a receiver cross branch including asecond p-i-n diode coupling said receiver terminal to a positivepotential source, said receiver cross branch including a resistorcoupling said receiver terminal to circuit ground and said positivepotential source is in the form of a voltage divider having a pair ofresistors series connecting a potential source to a potential referencelevel.